Typically, rotary wing aircrafts like helicopters are sustained by a rotor, rotating about a vertical rotor shaft, generating lift or upward thrust. In a conventional helicopter the thrust from the rotor can be controlled by changing the pitch angle (or in short; the pitch) of the rotor blades. The pitch is in the field of propeller aerodynamics defined as the lateral angle between the blades and a reference plane perpendicular to the rotor shaft axis.
By collectively changing the pitch of all the rotor blades or by changing the rotational speed of the rotor, the helicopter can be controlled in the vertical direction. The horizontal direction of flight and the stability of the helicopter, however, is controlled by cyclically adjusting the pitch of individual blades. Cyclically adjusting the pitch means that the pitch of each rotor blade is adjusted from a maximum in a particular position of rotation to a minimum at the opposite side. This causes the lift in one part of the rotation to be larger than in other parts, whereby the rotor is tilted with respect to the reference plane. When the rotor tilts like this, the initially vertical thrust also tilts, and therefore gets a horizontal component that pulls the helicopter in the desired direction.
Normally, a helicopter must be actively controlled by a well trained pilot or from gyroscopic sensors and computers. The necessary means to varying and controlling the pitch angle of each blade are normally complicated, expensive and add weight to the helicopter.
Published U.S. application US2004/0245376 to the present inventor discloses a rotor and a coaxial rotor system that enables passively stable hover. The rotor system disclosed in this publication is capable of generating the required lift to enable an aircraft to have sustained stable flight including passively stable hover without the need for active control of the rotor.
In the preferred embodiment disclosed in the above mentioned application the rotor consists of 4 rotor blades arranged in two pairs or sets. The rotor blades are fixed to two center pieces; two rotor blades extending in opposite direction fixed to an upper center piece and the other two rotor blades oriented 90 degrees with respect to the first ones, fixed to a lower center piece. At their tip the blades are fixed to a ring encircling the whole rotor. This ring, per definition, lies in what is normally called the rotating plane of the rotor. Each orthogonally oriented center piece is connected to the vertical rotor shaft with a hinge having axis perpendicular to the longitudinal direction of the rotor blades.
The hinged connection enables the torque from the rotor shaft to spin the rotor while it at the same time allows each set of rotor blades to flap (one blade tilts up while the opposite blade tilts down). The pitch of the inner part of all the rotor blades remains fixed when the rotor tilts, but the pitch of the tip of the rotor blades, however, follow the tilting of the ring. This implies that the rotor blades must be flexible and that one set of blades must twist about their longitudinal axis when the other set of blades flaps up and down.
The rotor disclosed in the above mentioned publication rely on three different means functioning together to control the stability and behavior of the rotor.
Firstly, the hinged connection between each set of rotor blades and the rotor shaft, together with the flexible blades, enables the rotating plane to tilt more or less freely in any direction with respect to the reference plane. This is important to prevent any gyroscopic coupling between the rotor and the helicopter that would otherwise lead to instability of the whole aircraft.
Secondly, the rotor is stabilized with respect to the rotor shaft and the aircraft by keeping the inner part of the rotor blades at a fixed pitch relative to the reference plane. If the rotor and the rotating plane have been tilted, the blades will in effect follow an up-and-down path as they rotate, resulting in different lift in different parts of the rotation, whereby the rotor is tilted back to its initial position by aerodynamic forces. Due to the same effect the rotor will also follow any tilting of the aircraft, thereby enabling horizontal flight by tilting the whole aircraft by means of weight shifting or by a vertical thrust from a small propeller at the back of the aircraft.
Thirdly, in a rotor that moves horizontally the rotor blades will have different relative airspeed depending on where in the rotation they are. In the part of rotation where the blades rotate forward in the same direction as the movement, the relative airspeed increases. The increased speed gives increased lift that starts to tilt the rotor or more precisely the rotating plane up in the front. When the rotating plane tilts, the vertical thrust also tilts and gets a horizontal component acting against the movement, trying to stop it. The fixed pitch relative to the rotating plane (the ring) is important because it very much adds to the tilting tendency and it ensures that even small movements with respect to the surrounding air, will tilt the rotating plane and stop the movement of the aircraft.
When the movement stops, the second means stabilizing the rotor with respect to the rotor shaft, brings the rotor back to its initial position. This third feature reduces the maximum forward speed of the aircraft but on the other hand makes it more stable.
This rotor technology has been used in e.g. different toy helicopters. It is simpler than other helicopter rotors and it enables a remotely controlled, fully maneuverable and passively stable helicopter.
However, the rotor described above consists of several parts and needs careful assembly to function as intended. The number of parts and the possible difficulties during assembly increases the production cost and might give quality problems. Each of the two rotors in the toy helicopters mentioned above consists of 20 smaller or larger parts.
The rotors must be assembled by trained workers at a factory and therefore need to be shipped assembled and ready for use. This requires a relative large box protecting them.
While still utilizing the same basic rotor concept, a different mechanical approach could drastically reduce the number of parts and simplify the assembly. Such a new mechanical solution could also enable the rotor to be folded up for easy shipment.